SIZE REFUGIA FROM PREDATION THROUGH TIME IN AN ATRYPIDE BRACHIOPOD LINEAGE FROM THE MIDDLE DEVONIAN OF MICHIGAN

Alpena and Presque Isle Counties of Michigan, USA contain well preserved brachiopods from Devonian strata that have never been examined for predatory traces. Predation scars may offer insight into ancient predator-prey relationships and evolutionary processes. One approach to studying predation in fossils is through repair frequencies, which indicate a failed attempt at predation as the prey survives the attack and repairs its shell. A change in repair frequency may indicate either a change in the number of attacks on the prey or a change in the success of the predator. Size refugia, the size of the prey at which the predator will no longer take the prey, can be used to differentiate between these two possibilities. The goal of this study is to examine predatory traces and size refugia in a single lineage of atrypide brachiopods to determine the relative performance of predators to prey through time in the Middle Devonian of Michigan.

1064 specimens of Desquamatia, on loan from the University of Michigan, were reviewed for crushing predation through multiple shales in the Middle Devonian. The units, from base upward, are Bell Shale, Ferron Point Fm., Genshaw Fm., Dock Street Clay, and Potter Farm Fm.; these shales were deposited in similar environments. Repair frequency (R%) was determined for each unit. Body size was measured for all specimens and the body size at the time of attack was measured for the specimens that had repair scars. The maximum size at attack for each unit was used as a proxy for size refuge.

The results showed a U-shaped trend in R% through time (min. = 8.87%, max. = 21.37%), with the lowest R% in the Genshaw. The size refuge was roughly inverse to R%, showing an N-shaped trend, with the greatest size refuge in the Genshaw. These results suggest that the adaptive difference between predators and prey is fluctuating through time: during Genshaw time, the predators grew more effective relative to their prey, as indicated by the predator’s ability to take larger prey, whereas pre- and post-Genshaw, the predators were less effective. This may imply that an evolutionary arms race occurred between predators and prey. Further research is needed to determine whether there was a change in morphology in the prey post-Genshaw in response to increased predation pressure.